Re: [Xen-devel] PCIe IOMMU ACS support

[Roman Shaposhnik <roman@xxxxxxxxxx>]

On Wed, Mar 25, 2020 at 4:05 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> Adding the PCI and IOMMU maintainers.
>
> On Mon, Mar 23, 2020 at 01:55:01PM -0700, Roman Shaposhnik wrote:
> > Hi!
> >
> > I was going through how Xen support PCIe IOMMU ACS and
> > all I could find is this:
> >     
> > https://github.com/xen-project/xen/blob/master/xen/drivers/passthrough/pci.c#L608
> > which looks to me as an attempt of enabling ACS opportunistically,
> > but still proceeding forward even if it fails.
>
> That's correct AFAICT. Xen will try to enable some features, but will
> proceed normally if ACS is not available, or if some of the features
> are not implemented.
>
> Are you looking to ensure that all devices on the system have a
> certain feature enabled?

My primary objective was to get some visibility into how Xen would
prevent two PCIe devices behind a common bridge from doing p2p
transactions (thus violating VM boundaries if those devices are
assigned to different domains).

It looks like Xen simply trusts the hardware.

> Can you provide some more details about what you expect of ACS
> handling?

I was actually surprised not to see IOMMU groups in the style of what
VFIO https://www.kernel.org/doc/Documentation/vfio.txt

Thanks,
Roman.